The present invention relates to a centrifugal compressor such as an exhaust gas turbine supercharger or the like, and more particularly to a diffuser device disposed in a passageway between an air outlet of an impeller and a swirl chamber within a casing in such a centrifugal compressor and a method for manufacturing the diffuser device.
One example of an essential structure on the blower side of an exhaust gas turbine supercharger in the prior art is illustrated in cross-section in FIG. 1. In the structure shown in FIG. 1, externally supplied fresh air is compressed by front blades 5 and an impeller 6 mounted on a rotor shaft 7 which is in turn driven by an exhaust gas turbine, and then an air flow having a pressure and a flow rate required by a diesel engine is formed by a diffuser device 4' and is supplied to a diesel engine through a swirl chamber formed by an outer volute casing 1 and an inner volute casing 2. The diffuser device 4' is fixedly secured to the inner volute casing 2 by means of bolts 20. The air flow supplied by the supercharger is matched with the pressure and flow rate required by the diesel engine generally by means of the diffuser device 4', front blades 5 and impeller 6, but it is a common practice to achieve fine adjustment by means of the diffuser device 4'. In the illustrated construction, since the diffuser device 4' has an integral structure, varieties of specifications required for the diffuser device 4' are so many that even if calculations matching are conducted with the highest class of electronic computer known at present, normally it is required to prepare diffuser devices 4' having two different specifications close to a desired specification. Increase of the construction time, cost and amount of storage due to the necessity for such diffuser devices will raise the overall cost of a supercharger. Moreover, there is a shortcoming that the preparation of two or more diffuser devices require a relatively large investment.